Method and device to feed premixed gas to tuyeres of electric arc furnaces

ABSTRACT

Method and device to feed tuyeres in an electric furnace, the tuyeres comprising a central conduit to deliver a comburent gaseous substance and a peripheral conduit to deliver a cooling gaseous substance, the method providing to send to the central conduit a comburent mixture consisting of oxygen and neutral gas (N 2 , Ar, etc.) in defined percentages, the oxygen and the neutral gas being fed, for each of the tuyeres, to a mixing device with a high mixing capacity, at least the pressure of the neutral gas being able to be adjusted on the relative feed line so as to modulate the pressure of the mixture according to the characteristics of the melting process.

FIELD OF APPLICATION

This invention concerns a method to feed tuyères in electric furnaces,and the relative feed device, as set forth in the respective mainclaims.

The invention is applied in electric melting furnaces, and particularlyin electric arc furnaces, to send a comburent mixture to the bottomtuyères provided in the hearth of the furnace.

The feed method according to the invention makes it possible to prolongthe working life of the tuyères and improve the productivity thereof,reducing wear and deterioration and optimising efficiency.

The invention also makes it possible to increase the effect of the heatcontribution which derives from the combustion of the substances insidethe furnace and the comburent mixture delivered through the tuyères.

STATE OF THE ART

In electric melting furnaces, whether they be fed with direct oralternating current, the state of the art covers the use of tuyères onthe bottom through which gassy substances with a high comburent powerare introduced inside the furnace, with the function of determining andintensifying combustion reactions with the chemical substances which areliberated inside the furnace.

These combustion reactions make it possible to recover the intrinsicpotential of energy possessed mainly by carbon and other chemicalsubstances liberated inside the furnace by means of an exothermicoxidation reaction.

These combustion reactions supply an auxiliary contribution of energywith respect to the main means which the furnace, thus allowing a greatsaving in energy, speeding up the melting cycles and improving thequality of the metal produced.

The tuyères normally comprise a conduit, with its mouth protrudinginside the furnace, connected to means to feed the comburent substance,normally oxygen or air enriched with oxygen.

Other gassy substances may also be sent through this conduit, such asazote, neutral gases or other similar substances, but one gas mustalways be separate from the other.

The gassy substance sent through the tuyères also has the function ofstirring the bath of liquid metal.

In order to reduce wear on the upper part of the tuyère which protrudesinto the furnace—wear caused by the enormous overheating due to thecombustion reactions—the state of the art provides to send a coolinggas, normally a mixture of hydrocarbons, azote, inert gases or similar,together with the comburent gas.

There have been proposals for tuyères wherein, peripherally to a centralconduit which delivers the comburent substance, an annular conduit isprovided which introduces the cooling mixture.

The present Applicant, in the European Patent application EP-A-625685,described a tuyère wherein the oxygen-delivery conduit is made of copperand is obtained by making a central hole in a polygonal or star-shapedprofile, the slits obtained at the side of the central hole being usedto send the cooling mixture.

The said patent application also describes how the melting cycle isdivided into a plurality of intervals, each of which is characterised byspecific parameters of pressure both of the comburent oxygen and also ofthe cooling mixture.

Moreover, the feed method of EP′685 provides to regulate the pressure ofthe oxygen by using suitable readers and transducers on the line offeed.

The consequent variations in the flow of oxygen influence the flow of atleast one of the components of the cooling mixture, the flow of theother component or components of the cooling mixture consequentlyvarying so as to ensure a constant and controlled feed to the mixingdevice which supplies the cooling mixture to the annular deliveryconduits inside the furnace.

By using this adjustment technique, together with the particularconformation of the tuyères, their size and their specific distributionon the bottom of the furnace, excellent results have been obtained interms of energy efficiency and the protection of the tuyères from wearand deterioration.

However, in practice, it has been seen that in the event that heavymaterial of a certain thickness is deposited above the outlet mouth ofthe tuyères, the comburent gassy substance is not able to perforate thisand therefore the tuyère rapidly deteriorates and is ruined.

This problem means that it is soon necessary to carry out maintenanceoperations or to replace the tuyères, which means interrupting thecycle, long down-times and a great deal of work for the workers.

These problems also cause periods when the tuyères are working at lowefficiency, with a consequent reduction in the energy supply and adeterioration in the quality of the molten metal produced.

The present Applicant has designed, tested and embodied this inventionto overcome the shortcomings of the state of the art, and to obtainfurther advantages.

DISCLOSURE OF THE INVENTION

The invention is set forth and characterised in the respective mainclaims, while the dependent claims describe variants of the idea of themain embodiment.

According to the invention, the central conduit of each tuyère is fedwith a mixture consisting of oxygen and a neutral gas such as azote orsimilar.

These two components are sent to a mixing device through relative linescomprising at least means to modulate the sending pressure.

According to a variant, the line to feed the neutral gas comprises atleast two send lines each of which defines a specific level of pressureat inlet.

The second send line makes it possible to obtain an oxygen-neutral gasmixture under pressure.

According to the invention, the mixing device which feeds the centralconduit of the tuyère is of the type which is suitable to create anincrease in the speed of the gas at inlet, so that at outlet the jet ofthe mixture has a kinetic energy able to perforate at least partly anypossible layer of material which has accumulated on the bottom.

According to a variant, the mixing device is of the type which issuitable to induce a high quantity of vortical motion in the mixture ofcomburent substances at outlet.

The present Applicant has verified that by using the mixture with theaforesaid components, and using a mixing device which develops a highspeed and in the gases at outlet, it is possible to prolong the workinglife of the tuyères.

According to another variant, the feed line of the cooling mixtureprovides at least a mixing system of a type which will achieve highspeed and vortical motion at outlet.

ILLUSTRATION OF THE DRAWINGS

The attached Figures are given as a non-restrictive example and show apreferential embodiment of the invention as follows:

FIG. 1 shows a diagram of the feed device according to the invention forthe comburent mixture sent to the central conduit of the tuyère;

FIG. 2a shows in cross section a possible mixing device applied to thedevice in FIG. 1;

FIG. 2b shows a view from “A” of FIG. 2a.

FIG. 3 shows a cross-sectional view of a tuyere described in Applicants'European Patent Application EP-625685.

DESCRIPTION OF THE DRAWINGS

With reference to the attached Figures, the reference number 10 denotesthe device according to the invention employed to feed the comburentmixture to the tuyères in electric arc furnaces.

The device 10 is used in association with tuyères of the type includinga central conduit to deliver the comburent mixture and peripheral orannular conduits through which a gassy cooling mixture passes.

The device 10 therefore acts in cooperation with a device to mix andfeed cooling gases which is not shown here and of a type substantiallyalready known to the state of the art.

The device 10 according to the invention feeds the central conduit ofthe tuyères with a comburent mixture formed of oxygen (O₂) and neutralgas (N₂, Ar, etc.) and comprises, in this case, a line to feed oxygen 11and a line to feed neutral gas 12 with a double sending branch, thefirst branch 12 a and the second branch 12 b, each of which defines arelative level of pressure at inlet.

The two sending branches 12 a, 12 b are selected singly according to thedesired pressure of the comburent mixture which is to be obtained.

In this case, the second sending branch 12 b of the line to feed neutralgas 12 makes it possible to obtain a comburent mixture with a greaterpressure (about 7 bar) than that obtained with the first sending branch12 a, which gives a comburent mixture with a pressure of about 3 bar.

This circuit with two branches 12 a and 12 b also makes it possible tomodulate the pressure of the neutral gas within a wider range than thetwo values mentioned above, using both inlets and modulating the flowsso as to mix the gases at different pressures, thus obtainingintermediate pressures between the two original pressures.

The feed lines 11 and 12 are associated at their terminal ends to amixing device 13 achieving the mixture of oxygen and neutral gas beforethey are sent to the tuyères.

FIGS. 2a and 2 b show a possible mixing device 13 employed by the device10 according to the invention.

On the inlet side 13 a, the mixing device 13 includes a plurality ofinlet holes 14, of limited section, arranged on the periphery of thelongitudinal axis “y”, by means of which the oxygen and neutral gasenter into a first inner seating 15 a at a speed which is alreadygreater than the inlet speed.

The first inner seating 15 a communicates with a second inner seating 15b by means of an axial channel 16 with a raised edge 16 a.

At its centre, and therefore in a position substantially aligned withthe axial channel 16, the second inner seating 15 b has a conical hollow18 and on the periphery a plurality of delivery holes 19, in this casesix in number, arranged on the outlet side 13 b of the mixing device 13.

The limited-section delivery holes 19 cause a further acceleration and ahigh vortical motion in the mixture obtained at it emerges.

This conformation of the mixing device 13 therefore causes vorticalflows 17 of oxygen-neutral gas to form, thus ensuring an efficientmixing and a thorough mingling of the two gases.

This makes it easier to perforate any material possibly obstructing theoutlet mouth of the tuyères and increases the efficiency and thedelivery of energy of the tuyères.

According to a variant which is not shown here, the mixing device 13 isof the type which is suitable to generate at outlet an even greatervortical motion in the comburent mixture delivered.

The line to feed oxygen 11 and each of the two sending branches 12 a and12 b comprise an initial segment, respectively 111, 112 a and 112 b,which is common to all the tuyères and a final segment, respectively211, 212 a and 212 b, which develops parallel for each individualtuyère.

In this case, in its initial segment 111, the line to feed oxygen 11includes in series a manual valve 20, a pressure regulator 21cooperating with a manometer 24, a servo-commanded valve 22 and adischarge valve 23.

The final segment 211 of the line to feed oxygen 11 comprises, in seriesand for each tuyère, a manual valve 25, a servo-commanded valve 26, anon-return valve 27, a regulator valve 28, a flow meter 29 and a manualvalve 30 located upstream of the mixing device 13.

Each of the initial segments 112 a and 112 b of the sending branches 12a, 12 b of the line 12 to feed the neutral gas comprise, arranged inline, a manual valve, respectively 31 a and 31 b, a pressure regulator,32 a and 32 b, a manometer, 33 a and 33 b, and a non-return valve, 34 aand 34 b.

In both the final segments 212 a and 212 b, for each tuyère there is amanual valve, respectively 35 a and 35 b, arranged upstream of a matingservo-commanded valve 36 a and 36 b.

The final segment 212 a of the first branch 12 a also includes adischarge valve 37, while the final segment 212 b of the second branch12 b comprises a regulator valve 38.

The part shared by the two sending branches 12 a, 12 b of the line 12 tofeed the neutral gas includes in series a servo-commanded valve 39, adevice to measure the flow 40 and a non-return valve 41.

In a preferential embodiment of the invention, the device 10 is governedby a control unit by means of which, by setting certain parametersconcerning the comburent mixture to be obtained, and the delivery timesthereof, the activation cycles of the feeding lines 11 and 12 areautomatically controlled so as to obtain a comburent mixture with thedesired chemical/physical characteristics.

This setting of the parameters can also be determined by the controlunit according to the characteristics of the melting cycle, the rawmaterial, the quality of the steel to be obtained, etc.

The automatic control of the feeding lines 11 and 12 makes it possible,for every interval in the melting cycle, to regulate the pressure atwhich the gasses are sent to the mixing device 13, and also the relativeflow, so that characteristics of energy supply can be maximisedaccording to the specific melting process which is being carried out inthe furnace.

What is claimed is:
 1. Method to feed tuyeres in an electric furnace tocarry out a melting process, the tuyeres comprising a central conduit todeliver a comburent gaseous substance and a peripheral conduit todeliver a cooling gaseous substance, the method comprising feedingoxygen and neutral in defined percentages to a mixing device with a highmixing capacity, adjusting at least a pressure of the neutral gas so asto modulate a pressure of the mixture according to characteristics ofthe melting process, and sending a comburent mixture comprising theoxygen and neutral gas in defined percentages to the central conduit foreach of the tuyeres.
 2. Method as in claim 1, wherein the neutral gas isfed through at least two lines, a first low-pressure line and a secondhigh-pressure line.
 3. Method as in claim 2, wherein the at least twolines are used selectively and alternatively.
 4. Method as in claim 2,wherein the at least two lines are used contemporaneously and apercentage flow of each of the at least two lines being regulated so asto obtain a range of intermediate pressures between a high and lowpressure of each of the at least two lines.
 5. Method as in claim 2,wherein the second high-pressure line delivers neutral gas at 7 bar. 6.Method as claim 2, wherein the first low-pressure line delivers neutralgas at 3 bar.
 7. Device to feed a comburent gaseous substance to atuyere for an electric furnace, the device being comprising at least aline to feed oxygen and at least a line to feed neutral gas, the line tofeed oxygen and at least a line to feed neutral gas being connecteddownstream to a mixing device with a high mixing capacity, arrangedimmediately upstream of the tuyere, each of the line to feed oxygen andat least a line to feed neutral gas comprising at least means tomodulate and stabilize a pressure of delivery.
 8. Device as in claim 7,which comprises at least two lines to feed the neutral gas, ahigh-pressure line and a low-pressure line which converge at a positionupstream of the mixing device.
 9. Device as in claim 8, which comprisesmeans to autonomously regulate the flow of the at least two lines tofeed neutral gas according to a desired modulation of pressure of theneutral gas supplied to the mixing device.
 10. Device as in claim 7,wherein the mixing device is of the type suitable to generate a highspeed and vortical motion of the comburent gaseous substance delivered.11. Device as in claim 7, which comprises a control unit which regulatesat least flow and pressure of the comburent gaseous substance accordingto the raw material and the quality of the steel to be produced. 12.Method as in claim 1, wherein the neutral gas is selected from the groupconsisting of N₂ and Ar.